1. Field of the Invention
The invention generally relates to the manufacturing of thin and ultrathin glasses. In particular, the invention relates to the preparation of long ribbon-shaped thin and ultrathin glass films in the form of a rolled glass ribbon.
2. Description of Related Art
Thin glass is increasingly employed for various applications, such as in the sectors of consumer electronics, for example as cover glasses for semiconductor modules, for organic LED light sources, or for thin or curved display devices, or in sectors of renewable energy or energy technology, such as in solar cells. Examples include touch panels, capacitors, thin film batteries, flexible circuit boards, flexible OLEDs, flexible photovoltaic modules, and even e-papers. Thin glass is getting more and more into the focus for many applications due to its excellent properties such as chemical resistance, thermal shock resistance, and heat resistance, gas tightness, its high electrical insulation capability, matched expansion coefficients, flexibility, high optical quality, and light transmission, as well as high surface quality with very low roughness in case of a fire-polished surface of the two thin glass faces. In the present context, thin glass refers to glass films with thicknesses smaller than 1.2 mm to thicknesses of 5 μm and smaller. Due to its flexibility, thin glass in the form of a glass film is increasingly rolled up after production and stored in the form of a glass roll or transported for being cut into final size or further processed. In a roll-to-roll process, the glass film can furthermore be rewound and provided for further utilization after an intermediate treatment, such as coating or surface finishing. Compared to storage and transport of material in the form of flat sheets, the rolling up of the glass involves the advantage of a more cost-effective compact storage, transport and handling during further processing.
In further processing, smaller glass film portions are separated from the glass roll or else from material stored or transported in the form of flat sheets, according to the requirements. In some applications, these glass film portions are again used as bent or rolled glass.
With all its excellent properties, glass as a brittle material has a rather low breaking strength since it is less resistant to tensile stress. When glass is bent, tensile stresses will occur at the outer surface of the bent glass. For storage without breakage and for transport without breakage of such a glass roll or for utilization of smaller glass film portions without cracks and fractures, the quality and integrity of the edges is first of all important in order to avoid the formation of a crack or fracture in the wound or bent glass film. Damages at the edges, such as tiny cracks, e.g. microscopic cracks, may alone be the reason and point of origin for larger cracks or fractures in the glass film. Furthermore, due to the tensile stress at the upper surface of the rolled or bent glass film, the surface has to be undamaged and free of scratches, grooves or other surface defects in order to avoid the formation of a crack or fracture in the wound or bent glass film. Thirdly, internal tensions in the glass resulting from the production process should also be as low as possible or not existent in order to avoid the formation of a crack or fracture in the wound or bent glass film. In particular the quality of the edge of the glass film is of special importance with regard to crack formation or crack propagation up to the breaking of the glass film.
According to the prior art, thin glasses or glass films are mechanically scored and broken by a specially cut diamond or a small wheel made of special steel or tungsten carbide. In this case, a stress is selectively generated in the glass by the scoring of the surface. Along the so produced fissure, the glass is broken in controlled manner by pressure, tension, or bending. As a result, edge faces with strong roughness, many microscopic cracks and chipping or spalling defects will usually be produced at the peripheral edges.
In order to increase edge strength, edges can then be seamed, chamfered, or ground and polished. However, in the case of glass films in particular in a thickness range of less than 200 μm, mechanical processing of the edges can no longer be realized without an additional risk of cracking and breaking of the glass. Depending on the quality of the glass and in particular of the glass edge, different bending strengths of the glass are resulting. It is precisely the quality of the edge that is decisive in this case. Therefore, two glass films similar in material and surface quality may have very different breakage probabilities upon bending of the glass edge when the edges are of different quality. If the produced thin glass film is intended to be provided in rolled-up form, there is therefore a great uncertainty concerning the possible diameter of the roll core or of the inner diameter of the glass roll. The innermost glass layer of the glass roll, forming the inner surface of the glass roll, has the smallest bending radius and therefore is subjected to the greatest bending stress. The further outside the respective glass layer is located, the greater the bending radius will be. However, a major portion of the glass ribbon will typically have bending radii similar to the inner radius of the glass roll. In order to minimize the risk of breakage and/or to maximize the service life of the glass ribbon, it is, of course, always possible to choose a large roll core diameter. However, this has the disadvantage that very bulky dimensions are obtained in this way. For both the further processing of glass rolls and its storage, particularly compact dimensions, that means in particular small inner diameters, would of course be desirable. At the same time, however, the lowest possible breakage rate of the glass ribbon within a predetermined storage period should be provided.